Explanation:

Delhi first became the capital of a kingdom under the Tomara Rajputs, who were defeated in the middle of the twelfth century by the Chauhans (also referred to as Chahamanas) of Ajmer.
It was under the Tomaras and Chauhans that Delhi became an important commercial centre. Coins minted here, called dehliwal, had a wide circulation.
The transformation of Delhi into a capital that controlled vast areas of the subcontinent started with the foundation of the Delhi Sultanate in the beginning of the thirteenth century